PROJECT SUMMARY/ABSTRACT Unanticipated exposure to ionizing radiation (IR) is a clear and present risk due to unpredictable natural catastrophes near nuclear power plants and heightened global terrorist activities, as well as to medical radiological errors and machine malfunctions. Currently, no effective method exits to treat the potentially thousands of victims during a radiological event. The ideal treatment would be one that could be administered at least 24 hrs after exposure as a pill or injectable. A major obstacle to designing such a pharmaceutical is the lack of a molecular pathway to specifically target. A recent high-throughput screening (HTS) discovery project of small-molecule libraries uncovered a family of promising radiomitigators possessing a nitrophenylsulfonamide (NPS) core. Unfortunately, their mechanism of action in mitigating radiation toxicity is unknown. Serendipitously during an unrelated study on glutathione (GSH) efflux from irradiated cells via multidrug resistance transport (MRT), it was realized that inhibitors of MRT share the NPS chemical structure found in the screened radiomitigators. Because active efflux of GSH and GSH-conjugated molecules is a critical determinant of cell death, a novel idea arose connecting one study with the other. The hypothesis to be tested in this proposal is that inhibition of MRT mediated GSH efflux following IR will rescue cells from death, mitigating acute radiation toxicity. The radiomitigating NPS drugs selected from the HTS and known MRT inhibitors will be used to investigate the following specific aims: 1) Establish importance of GSH efflux and cell death kinetics post-IR, 2) Establish MRT as a mediator of IR-induced cell death and as a target for mitigation, 3) Optimize administration of GSH efflux/MRT inhibitor 24h or more after irradiation, and 4) determine differential effects of GSH efflux/MRT inhibitors on hematopoietic and gastrointestinal tissues to target mitigation of organ-specific radiation syndromes. The milestones will be the identification of a link between MRT-mediated GSH efflux and radiation-induced death, discovery of a set of unique tissue-specific targets for mitigating acute radiation toxicity, and establishment of a basis to develop accessible pharmaceuticals for wide dispersal during a catastrophic radiological event.